[CentOS] Starting a Local CentOS Mirror

Hi all,

I would like to set up a local CentOS mirror, as suggested by others
on this list, to cut down on bandwidth usage, etc. I have read a
couple HowTos and it seems pretty straight-forward, but I do have some
questions.

1) How should I change my entries in /etc/yum.repos.d? Specifically,
should I replace CentOS-Base.repo with my own .repo file?

2) Is there a way to configure yum to prefer the local repo, but use
an external mirror if the local is not available?

3) What is the best practice around keeping the local repo updated? A
once-a-day cron job to rsync from an external mirror? If this is the
correct approach, is there a way to rsync from a "mirror list" so that
updates won't fail if a particular mirror happens to be unavailable
when the cron job runs?

Thanks in advance for any guidance you can provide. I want to be sure
I get this right!